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"Artificial sweeteners linked to risk of weight gain," the Daily Mirror reports. Researchers looking at data gathered in previous studies reported a link between artificial sweeteners – ironically often associated with diet drinks – and weight gain. They also found a link with type 2 diabetes, high blood pressure and stroke.

However, the results of this review should be treated with caution. They are based on seven small, low-quality trials, and 30 cohort studies, which cannot show cause and effect. There was such a wide difference in the results and methods between the trials that pooling them increases the risk the results could have occurred by chance.

Where did the story come from?

The study was carried out by researchers from various hospitals and institutions in Canada, including the University of Manitoba. It did not receive any specific funding. The study was published in the peer-reviewed Canadian Medical Association Journal.

Neither The Independent nor the Daily Mirror explained any of the limitations in the underlying studies or recognised that pooling the results of such different types of studies increases the possibility that the results occur by chance.

The Mirror's claim that the study found artificial sweeteners could affect gut bacteria and appetite is inaccurate. Researchers speculated along these lines, but these factors were not included in the research.

What kind of research was this?

This was a systematic review of published research on the effect of artificial sweeteners on body mass index (BMI) and a range of medical conditions. The results of any relevant randomised controlled trials and cohort studies were pooled in a meta-analysis. This type of review is useful for collating a large amount of information, but the findings rely on the quality and strength of the underlying evidence.

What did the research involve?

The researchers searched three medical databases for relevant trials and cohort studies. After sifting through more than 11,000 article titles, they found seven randomised controlled trials and 30 cohort studies that looked at the consumption of artificial sweeteners and various outcome measures.

The trials included adults who were overweight, obese or had high blood pressure. They were randomised to consume either a non-nutritive sweetener, such as aspartame, taken as a capsule or in "diet drinks", or placebo or water daily for 6 to 24 months.

The cohort studies included between 347 and 97,991 adults whose weight ranged from healthy to obese. The researchers grouped adults into highest and lowest sweetener consumption, mostly from fizzy drinks. They then compared any change in weight or BMI, or development of type 2 diabetes or cardiovascular disease over a follow-up period ranging from 9 months to 38 years.

What were the basic results?

According to the randomised controlled trials:

Sweeteners did not have any effect on BMI (mean difference 0.37kg/m2, 95% confidence interval [CI] 1.10 to 0.36). This was based on three similar trials with 242 people.

Sweeteners did not have an effect on weight change (mean difference 0.17kg, 95% CI 0.54 to 0.21). Five studies of 791 adults were included, though there were major differences between the studies.

The cohort studies found that compared with those who consumed the least sweetener, those who consumed the most had a:

The cohort studies also found that compared with those who did not consume sweeteners at all, high consumers of sweeteners had a slight increase in BMI, obesity and waist circumference.

How did the researchers interpret the results?

The researchers concluded that evidence from the trials "does not clearly support the intended benefits of non-nutritive sweeteners for weight management", and the cohort studies suggest "routine consumption of non-nutritive sweeteners may be associated with a long-term increase in BMI and elevated risk of cardiometabolic disease". However, they say these results are tentative and need to be confirmed in higher-quality trials.

Conclusion

The study authors suggest artificial sweeteners may not aid weight loss, despite marketing claims to the contrary, and could actually increase the risk of type 2 diabetes. However, the results need to be treated with caution, as this review had numerous limitations:

The randomised controlled trials had great variability and few participants, increasing the possibility of the results occurring by chance. They were also judged to be at a high risk of bias – for example, the participants could not be blinded to the intervention, and adherence (drop-out) rates were not provided.

We do not know whether there were any other interventions, such as change in diet or exercise, in either group over the course of the trials. Some trials involved consuming an artificial sweetener capsule, but we do not know what other drinks – "diet", sugary or alcoholic – were also being consumed. It is unlikely that changing one dietary factor would result in major weight reduction.

The cohort studies relied on food-frequency questionnaires. Poor recall can make these inaccurate, and they may not adequately account for changes in people's diet over time.

Cohort studies can be useful for looking at trends in large groups, but they cannot account for all possible confounding factors. Most of the studies only controlled for age, sex, smoking and physical activity level.

The majority of cohort studies were from the US, with only one cohort study from the UK, so the results may not be generalisable to the UK population.

There was too much variability between the cohort studies – such as different outcome measures, type of sweetener and length of study – to pool the results.

In summary, although this was a reasonably thorough review, it does not provide firm conclusions as to the beneficial or potentially harmful effects of artificial sweeteners. This is not the fault of the researchers but is down to the lack and poor quality of available evidence.

If you are overweight or obese, the best way to lose weight is by combining dietary changes with more exercise.